The goal of this proposal is to perform a series of experiments to test hypotheses centered on previous findings suggesting that adult mammalian skeletal muscle fibers exist as two distinct fast-and slow-types with regard to myosin head' chain (MHC) expression: l) a primary-slow type and a primary-fast type(s) with little or no capacity to change MHC phenotype in response to physiological stimuli; and 2) a secondary-fast and secondary-slow type(s), possessing the capacity to be maintained in either an untransformed or transformed (adapted) state, i.e., having the capacity to change MHC phenotype in response to physiological stimuli, but within a rather narrow range (slow type l to fast type IIa and fast type IIb to fast type IIx/IIa and vice versa). In testing this working hypothesis, several experiments are designed to determine more specifically the following: a) whether the combination of denervation and hypothyroidism serves as a regulatory signal to maintain or revert the secondary fast and slow fibers to their respective primary (default) state of MHC expression, so that the relative pool size of primary and secondary fibers can be determined for a given muscle; b) whether the combined actions of thyroid state (hypothyroidism vs hyperthyroidism) and altered mechanical activity (functional overload, hindlimb unloading etc.) act in parallel on similar pools of secondary fibers in regulating the adaptations in MHC expression in certain secondary-fast (IIa/IIx ) and secondary-slow (type I) fiber-types, thus suggesting a common pathway in the regulation of MHC adaptation; and c) whether, in those fibers sensitive to either or both thyroid state and mechanical loading, the adaptations in MHC phenotype involving the type I (slow) myosin are mediated via changes in the level of thyroid hormone receptors, thyroid receptor auxiliary proteins, and nuclear enhancer protein factors, which collectively regulate transcription of the type I MHC gene. Thus, these experiments should enhance our understanding concerning 1) why there is only a limited pool of fibers capable of changing phenotype in response to either thyroid hormone and/or mechanical intervention; 2) the subcellular events likely involved in transforming MHC phenotype in adaptive fibers sensitive to hormonal and mechanical intervention; and 3) the delineation of the limits whereby adaptations in MHC phenotype can occur to impact on muscle performance.